1 // SPDX-License-Identifier: GPL-2.0-or-later
2 /*
3 * transition.c - Kernel Live Patching transition functions
4 *
5 * Copyright (C) 2015-2016 Josh Poimboeuf <jpoimboe@redhat.com>
6 */
7
8 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
9
10 #include <linux/cpu.h>
11 #include <linux/stacktrace.h>
12 #include "core.h"
13 #include "patch.h"
14 #include "transition.h"
15
16 #define MAX_STACK_ENTRIES 100
17 #define STACK_ERR_BUF_SIZE 128
18
19 #define SIGNALS_TIMEOUT 15
20
21 struct klp_patch *klp_transition_patch;
22
23 static int klp_target_state = KLP_UNDEFINED;
24
25 static unsigned int klp_signals_cnt;
26
27 /*
28 * This work can be performed periodically to finish patching or unpatching any
29 * "straggler" tasks which failed to transition in the first attempt.
30 */
klp_transition_work_fn(struct work_struct * work)31 static void klp_transition_work_fn(struct work_struct *work)
32 {
33 mutex_lock(&klp_mutex);
34
35 if (klp_transition_patch)
36 klp_try_complete_transition();
37
38 mutex_unlock(&klp_mutex);
39 }
40 static DECLARE_DELAYED_WORK(klp_transition_work, klp_transition_work_fn);
41
42 /*
43 * This function is just a stub to implement a hard force
44 * of synchronize_rcu(). This requires synchronizing
45 * tasks even in userspace and idle.
46 */
klp_sync(struct work_struct * work)47 static void klp_sync(struct work_struct *work)
48 {
49 }
50
51 /*
52 * We allow to patch also functions where RCU is not watching,
53 * e.g. before user_exit(). We can not rely on the RCU infrastructure
54 * to do the synchronization. Instead hard force the sched synchronization.
55 *
56 * This approach allows to use RCU functions for manipulating func_stack
57 * safely.
58 */
klp_synchronize_transition(void)59 static void klp_synchronize_transition(void)
60 {
61 schedule_on_each_cpu(klp_sync);
62 }
63
64 /*
65 * The transition to the target patch state is complete. Clean up the data
66 * structures.
67 */
klp_complete_transition(void)68 static void klp_complete_transition(void)
69 {
70 struct klp_object *obj;
71 struct klp_func *func;
72 struct task_struct *g, *task;
73 unsigned int cpu;
74
75 pr_debug("'%s': completing %s transition\n",
76 klp_transition_patch->mod->name,
77 klp_target_state == KLP_PATCHED ? "patching" : "unpatching");
78
79 if (klp_transition_patch->replace && klp_target_state == KLP_PATCHED) {
80 klp_unpatch_replaced_patches(klp_transition_patch);
81 klp_discard_nops(klp_transition_patch);
82 }
83
84 if (klp_target_state == KLP_UNPATCHED) {
85 /*
86 * All tasks have transitioned to KLP_UNPATCHED so we can now
87 * remove the new functions from the func_stack.
88 */
89 klp_unpatch_objects(klp_transition_patch);
90
91 /*
92 * Make sure klp_ftrace_handler() can no longer see functions
93 * from this patch on the ops->func_stack. Otherwise, after
94 * func->transition gets cleared, the handler may choose a
95 * removed function.
96 */
97 klp_synchronize_transition();
98 }
99
100 klp_for_each_object(klp_transition_patch, obj)
101 klp_for_each_func(obj, func)
102 func->transition = false;
103
104 /* Prevent klp_ftrace_handler() from seeing KLP_UNDEFINED state */
105 if (klp_target_state == KLP_PATCHED)
106 klp_synchronize_transition();
107
108 read_lock(&tasklist_lock);
109 for_each_process_thread(g, task) {
110 WARN_ON_ONCE(test_tsk_thread_flag(task, TIF_PATCH_PENDING));
111 task->patch_state = KLP_UNDEFINED;
112 }
113 read_unlock(&tasklist_lock);
114
115 for_each_possible_cpu(cpu) {
116 task = idle_task(cpu);
117 WARN_ON_ONCE(test_tsk_thread_flag(task, TIF_PATCH_PENDING));
118 task->patch_state = KLP_UNDEFINED;
119 }
120
121 klp_for_each_object(klp_transition_patch, obj) {
122 if (!klp_is_object_loaded(obj))
123 continue;
124 if (klp_target_state == KLP_PATCHED)
125 klp_post_patch_callback(obj);
126 else if (klp_target_state == KLP_UNPATCHED)
127 klp_post_unpatch_callback(obj);
128 }
129
130 pr_notice("'%s': %s complete\n", klp_transition_patch->mod->name,
131 klp_target_state == KLP_PATCHED ? "patching" : "unpatching");
132
133 klp_target_state = KLP_UNDEFINED;
134 klp_transition_patch = NULL;
135 }
136
137 /*
138 * This is called in the error path, to cancel a transition before it has
139 * started, i.e. klp_init_transition() has been called but
140 * klp_start_transition() hasn't. If the transition *has* been started,
141 * klp_reverse_transition() should be used instead.
142 */
klp_cancel_transition(void)143 void klp_cancel_transition(void)
144 {
145 if (WARN_ON_ONCE(klp_target_state != KLP_PATCHED))
146 return;
147
148 pr_debug("'%s': canceling patching transition, going to unpatch\n",
149 klp_transition_patch->mod->name);
150
151 klp_target_state = KLP_UNPATCHED;
152 klp_complete_transition();
153 }
154
155 /*
156 * Switch the patched state of the task to the set of functions in the target
157 * patch state.
158 *
159 * NOTE: If task is not 'current', the caller must ensure the task is inactive.
160 * Otherwise klp_ftrace_handler() might read the wrong 'patch_state' value.
161 */
klp_update_patch_state(struct task_struct * task)162 void klp_update_patch_state(struct task_struct *task)
163 {
164 /*
165 * A variant of synchronize_rcu() is used to allow patching functions
166 * where RCU is not watching, see klp_synchronize_transition().
167 */
168 preempt_disable_notrace();
169
170 /*
171 * This test_and_clear_tsk_thread_flag() call also serves as a read
172 * barrier (smp_rmb) for two cases:
173 *
174 * 1) Enforce the order of the TIF_PATCH_PENDING read and the
175 * klp_target_state read. The corresponding write barrier is in
176 * klp_init_transition().
177 *
178 * 2) Enforce the order of the TIF_PATCH_PENDING read and a future read
179 * of func->transition, if klp_ftrace_handler() is called later on
180 * the same CPU. See __klp_disable_patch().
181 */
182 if (test_and_clear_tsk_thread_flag(task, TIF_PATCH_PENDING))
183 task->patch_state = READ_ONCE(klp_target_state);
184
185 preempt_enable_notrace();
186 }
187
188 /*
189 * Determine whether the given stack trace includes any references to a
190 * to-be-patched or to-be-unpatched function.
191 */
klp_check_stack_func(struct klp_func * func,unsigned long * entries,unsigned int nr_entries)192 static int klp_check_stack_func(struct klp_func *func, unsigned long *entries,
193 unsigned int nr_entries)
194 {
195 unsigned long func_addr, func_size, address;
196 struct klp_ops *ops;
197 int i;
198
199 for (i = 0; i < nr_entries; i++) {
200 address = entries[i];
201
202 if (klp_target_state == KLP_UNPATCHED) {
203 /*
204 * Check for the to-be-unpatched function
205 * (the func itself).
206 */
207 func_addr = (unsigned long)func->new_func;
208 func_size = func->new_size;
209 } else {
210 /*
211 * Check for the to-be-patched function
212 * (the previous func).
213 */
214 ops = klp_find_ops(func->old_func);
215
216 if (list_is_singular(&ops->func_stack)) {
217 /* original function */
218 func_addr = (unsigned long)func->old_func;
219 func_size = func->old_size;
220 } else {
221 /* previously patched function */
222 struct klp_func *prev;
223
224 prev = list_next_entry(func, stack_node);
225 func_addr = (unsigned long)prev->new_func;
226 func_size = prev->new_size;
227 }
228 }
229
230 if (address >= func_addr && address < func_addr + func_size)
231 return -EAGAIN;
232 }
233
234 return 0;
235 }
236
237 /*
238 * Determine whether it's safe to transition the task to the target patch state
239 * by looking for any to-be-patched or to-be-unpatched functions on its stack.
240 */
klp_check_stack(struct task_struct * task,const char ** oldname)241 static int klp_check_stack(struct task_struct *task, const char **oldname)
242 {
243 static unsigned long entries[MAX_STACK_ENTRIES];
244 struct klp_object *obj;
245 struct klp_func *func;
246 int ret, nr_entries;
247
248 ret = stack_trace_save_tsk_reliable(task, entries, ARRAY_SIZE(entries));
249 if (ret < 0)
250 return -EINVAL;
251 nr_entries = ret;
252
253 klp_for_each_object(klp_transition_patch, obj) {
254 if (!obj->patched)
255 continue;
256 klp_for_each_func(obj, func) {
257 ret = klp_check_stack_func(func, entries, nr_entries);
258 if (ret) {
259 *oldname = func->old_name;
260 return -EADDRINUSE;
261 }
262 }
263 }
264
265 return 0;
266 }
267
klp_check_and_switch_task(struct task_struct * task,void * arg)268 static int klp_check_and_switch_task(struct task_struct *task, void *arg)
269 {
270 int ret;
271
272 if (task_curr(task) && task != current)
273 return -EBUSY;
274
275 ret = klp_check_stack(task, arg);
276 if (ret)
277 return ret;
278
279 clear_tsk_thread_flag(task, TIF_PATCH_PENDING);
280 task->patch_state = klp_target_state;
281 return 0;
282 }
283
284 /*
285 * Try to safely switch a task to the target patch state. If it's currently
286 * running, or it's sleeping on a to-be-patched or to-be-unpatched function, or
287 * if the stack is unreliable, return false.
288 */
klp_try_switch_task(struct task_struct * task)289 static bool klp_try_switch_task(struct task_struct *task)
290 {
291 const char *old_name;
292 int ret;
293
294 /* check if this task has already switched over */
295 if (task->patch_state == klp_target_state)
296 return true;
297
298 /*
299 * For arches which don't have reliable stack traces, we have to rely
300 * on other methods (e.g., switching tasks at kernel exit).
301 */
302 if (!klp_have_reliable_stack())
303 return false;
304
305 /*
306 * Now try to check the stack for any to-be-patched or to-be-unpatched
307 * functions. If all goes well, switch the task to the target patch
308 * state.
309 */
310 ret = task_call_func(task, klp_check_and_switch_task, &old_name);
311 switch (ret) {
312 case 0: /* success */
313 break;
314
315 case -EBUSY: /* klp_check_and_switch_task() */
316 pr_debug("%s: %s:%d is running\n",
317 __func__, task->comm, task->pid);
318 break;
319 case -EINVAL: /* klp_check_and_switch_task() */
320 pr_debug("%s: %s:%d has an unreliable stack\n",
321 __func__, task->comm, task->pid);
322 break;
323 case -EADDRINUSE: /* klp_check_and_switch_task() */
324 pr_debug("%s: %s:%d is sleeping on function %s\n",
325 __func__, task->comm, task->pid, old_name);
326 break;
327
328 default:
329 pr_debug("%s: Unknown error code (%d) when trying to switch %s:%d\n",
330 __func__, ret, task->comm, task->pid);
331 break;
332 }
333
334 return !ret;
335 }
336
337 /*
338 * Sends a fake signal to all non-kthread tasks with TIF_PATCH_PENDING set.
339 * Kthreads with TIF_PATCH_PENDING set are woken up.
340 */
klp_send_signals(void)341 static void klp_send_signals(void)
342 {
343 struct task_struct *g, *task;
344
345 if (klp_signals_cnt == SIGNALS_TIMEOUT)
346 pr_notice("signaling remaining tasks\n");
347
348 read_lock(&tasklist_lock);
349 for_each_process_thread(g, task) {
350 if (!klp_patch_pending(task))
351 continue;
352
353 /*
354 * There is a small race here. We could see TIF_PATCH_PENDING
355 * set and decide to wake up a kthread or send a fake signal.
356 * Meanwhile the task could migrate itself and the action
357 * would be meaningless. It is not serious though.
358 */
359 if (task->flags & PF_KTHREAD) {
360 /*
361 * Wake up a kthread which sleeps interruptedly and
362 * still has not been migrated.
363 */
364 wake_up_state(task, TASK_INTERRUPTIBLE);
365 } else {
366 /*
367 * Send fake signal to all non-kthread tasks which are
368 * still not migrated.
369 */
370 set_notify_signal(task);
371 }
372 }
373 read_unlock(&tasklist_lock);
374 }
375
376 /*
377 * Try to switch all remaining tasks to the target patch state by walking the
378 * stacks of sleeping tasks and looking for any to-be-patched or
379 * to-be-unpatched functions. If such functions are found, the task can't be
380 * switched yet.
381 *
382 * If any tasks are still stuck in the initial patch state, schedule a retry.
383 */
klp_try_complete_transition(void)384 void klp_try_complete_transition(void)
385 {
386 unsigned int cpu;
387 struct task_struct *g, *task;
388 struct klp_patch *patch;
389 bool complete = true;
390
391 WARN_ON_ONCE(klp_target_state == KLP_UNDEFINED);
392
393 /*
394 * Try to switch the tasks to the target patch state by walking their
395 * stacks and looking for any to-be-patched or to-be-unpatched
396 * functions. If such functions are found on a stack, or if the stack
397 * is deemed unreliable, the task can't be switched yet.
398 *
399 * Usually this will transition most (or all) of the tasks on a system
400 * unless the patch includes changes to a very common function.
401 */
402 read_lock(&tasklist_lock);
403 for_each_process_thread(g, task)
404 if (!klp_try_switch_task(task))
405 complete = false;
406 read_unlock(&tasklist_lock);
407
408 /*
409 * Ditto for the idle "swapper" tasks.
410 */
411 cpus_read_lock();
412 for_each_possible_cpu(cpu) {
413 task = idle_task(cpu);
414 if (cpu_online(cpu)) {
415 if (!klp_try_switch_task(task)) {
416 complete = false;
417 /* Make idle task go through the main loop. */
418 wake_up_if_idle(cpu);
419 }
420 } else if (task->patch_state != klp_target_state) {
421 /* offline idle tasks can be switched immediately */
422 clear_tsk_thread_flag(task, TIF_PATCH_PENDING);
423 task->patch_state = klp_target_state;
424 }
425 }
426 cpus_read_unlock();
427
428 if (!complete) {
429 if (klp_signals_cnt && !(klp_signals_cnt % SIGNALS_TIMEOUT))
430 klp_send_signals();
431 klp_signals_cnt++;
432
433 /*
434 * Some tasks weren't able to be switched over. Try again
435 * later and/or wait for other methods like kernel exit
436 * switching.
437 */
438 schedule_delayed_work(&klp_transition_work,
439 round_jiffies_relative(HZ));
440 return;
441 }
442
443 /* we're done, now cleanup the data structures */
444 patch = klp_transition_patch;
445 klp_complete_transition();
446
447 /*
448 * It would make more sense to free the unused patches in
449 * klp_complete_transition() but it is called also
450 * from klp_cancel_transition().
451 */
452 if (!patch->enabled)
453 klp_free_patch_async(patch);
454 else if (patch->replace)
455 klp_free_replaced_patches_async(patch);
456 }
457
458 /*
459 * Start the transition to the specified target patch state so tasks can begin
460 * switching to it.
461 */
klp_start_transition(void)462 void klp_start_transition(void)
463 {
464 struct task_struct *g, *task;
465 unsigned int cpu;
466
467 WARN_ON_ONCE(klp_target_state == KLP_UNDEFINED);
468
469 pr_notice("'%s': starting %s transition\n",
470 klp_transition_patch->mod->name,
471 klp_target_state == KLP_PATCHED ? "patching" : "unpatching");
472
473 /*
474 * Mark all normal tasks as needing a patch state update. They'll
475 * switch either in klp_try_complete_transition() or as they exit the
476 * kernel.
477 */
478 read_lock(&tasklist_lock);
479 for_each_process_thread(g, task)
480 if (task->patch_state != klp_target_state)
481 set_tsk_thread_flag(task, TIF_PATCH_PENDING);
482 read_unlock(&tasklist_lock);
483
484 /*
485 * Mark all idle tasks as needing a patch state update. They'll switch
486 * either in klp_try_complete_transition() or at the idle loop switch
487 * point.
488 */
489 for_each_possible_cpu(cpu) {
490 task = idle_task(cpu);
491 if (task->patch_state != klp_target_state)
492 set_tsk_thread_flag(task, TIF_PATCH_PENDING);
493 }
494
495 klp_signals_cnt = 0;
496 }
497
498 /*
499 * Initialize the global target patch state and all tasks to the initial patch
500 * state, and initialize all function transition states to true in preparation
501 * for patching or unpatching.
502 */
klp_init_transition(struct klp_patch * patch,int state)503 void klp_init_transition(struct klp_patch *patch, int state)
504 {
505 struct task_struct *g, *task;
506 unsigned int cpu;
507 struct klp_object *obj;
508 struct klp_func *func;
509 int initial_state = !state;
510
511 WARN_ON_ONCE(klp_target_state != KLP_UNDEFINED);
512
513 klp_transition_patch = patch;
514
515 /*
516 * Set the global target patch state which tasks will switch to. This
517 * has no effect until the TIF_PATCH_PENDING flags get set later.
518 */
519 klp_target_state = state;
520
521 pr_debug("'%s': initializing %s transition\n", patch->mod->name,
522 klp_target_state == KLP_PATCHED ? "patching" : "unpatching");
523
524 /*
525 * Initialize all tasks to the initial patch state to prepare them for
526 * switching to the target state.
527 */
528 read_lock(&tasklist_lock);
529 for_each_process_thread(g, task) {
530 WARN_ON_ONCE(task->patch_state != KLP_UNDEFINED);
531 task->patch_state = initial_state;
532 }
533 read_unlock(&tasklist_lock);
534
535 /*
536 * Ditto for the idle "swapper" tasks.
537 */
538 for_each_possible_cpu(cpu) {
539 task = idle_task(cpu);
540 WARN_ON_ONCE(task->patch_state != KLP_UNDEFINED);
541 task->patch_state = initial_state;
542 }
543
544 /*
545 * Enforce the order of the task->patch_state initializations and the
546 * func->transition updates to ensure that klp_ftrace_handler() doesn't
547 * see a func in transition with a task->patch_state of KLP_UNDEFINED.
548 *
549 * Also enforce the order of the klp_target_state write and future
550 * TIF_PATCH_PENDING writes to ensure klp_update_patch_state() doesn't
551 * set a task->patch_state to KLP_UNDEFINED.
552 */
553 smp_wmb();
554
555 /*
556 * Set the func transition states so klp_ftrace_handler() will know to
557 * switch to the transition logic.
558 *
559 * When patching, the funcs aren't yet in the func_stack and will be
560 * made visible to the ftrace handler shortly by the calls to
561 * klp_patch_object().
562 *
563 * When unpatching, the funcs are already in the func_stack and so are
564 * already visible to the ftrace handler.
565 */
566 klp_for_each_object(patch, obj)
567 klp_for_each_func(obj, func)
568 func->transition = true;
569 }
570
571 /*
572 * This function can be called in the middle of an existing transition to
573 * reverse the direction of the target patch state. This can be done to
574 * effectively cancel an existing enable or disable operation if there are any
575 * tasks which are stuck in the initial patch state.
576 */
klp_reverse_transition(void)577 void klp_reverse_transition(void)
578 {
579 unsigned int cpu;
580 struct task_struct *g, *task;
581
582 pr_debug("'%s': reversing transition from %s\n",
583 klp_transition_patch->mod->name,
584 klp_target_state == KLP_PATCHED ? "patching to unpatching" :
585 "unpatching to patching");
586
587 klp_transition_patch->enabled = !klp_transition_patch->enabled;
588
589 klp_target_state = !klp_target_state;
590
591 /*
592 * Clear all TIF_PATCH_PENDING flags to prevent races caused by
593 * klp_update_patch_state() running in parallel with
594 * klp_start_transition().
595 */
596 read_lock(&tasklist_lock);
597 for_each_process_thread(g, task)
598 clear_tsk_thread_flag(task, TIF_PATCH_PENDING);
599 read_unlock(&tasklist_lock);
600
601 for_each_possible_cpu(cpu)
602 clear_tsk_thread_flag(idle_task(cpu), TIF_PATCH_PENDING);
603
604 /* Let any remaining calls to klp_update_patch_state() complete */
605 klp_synchronize_transition();
606
607 klp_start_transition();
608 }
609
610 /* Called from copy_process() during fork */
klp_copy_process(struct task_struct * child)611 void klp_copy_process(struct task_struct *child)
612 {
613
614 /*
615 * The parent process may have gone through a KLP transition since
616 * the thread flag was copied in setup_thread_stack earlier. Bring
617 * the task flag up to date with the parent here.
618 *
619 * The operation is serialized against all klp_*_transition()
620 * operations by the tasklist_lock. The only exception is
621 * klp_update_patch_state(current), but we cannot race with
622 * that because we are current.
623 */
624 if (test_tsk_thread_flag(current, TIF_PATCH_PENDING))
625 set_tsk_thread_flag(child, TIF_PATCH_PENDING);
626 else
627 clear_tsk_thread_flag(child, TIF_PATCH_PENDING);
628
629 child->patch_state = current->patch_state;
630 }
631
632 /*
633 * Drop TIF_PATCH_PENDING of all tasks on admin's request. This forces an
634 * existing transition to finish.
635 *
636 * NOTE: klp_update_patch_state(task) requires the task to be inactive or
637 * 'current'. This is not the case here and the consistency model could be
638 * broken. Administrator, who is the only one to execute the
639 * klp_force_transitions(), has to be aware of this.
640 */
klp_force_transition(void)641 void klp_force_transition(void)
642 {
643 struct klp_patch *patch;
644 struct task_struct *g, *task;
645 unsigned int cpu;
646
647 pr_warn("forcing remaining tasks to the patched state\n");
648
649 read_lock(&tasklist_lock);
650 for_each_process_thread(g, task)
651 klp_update_patch_state(task);
652 read_unlock(&tasklist_lock);
653
654 for_each_possible_cpu(cpu)
655 klp_update_patch_state(idle_task(cpu));
656
657 /* Set forced flag for patches being removed. */
658 if (klp_target_state == KLP_UNPATCHED)
659 klp_transition_patch->forced = true;
660 else if (klp_transition_patch->replace) {
661 klp_for_each_patch(patch) {
662 if (patch != klp_transition_patch)
663 patch->forced = true;
664 }
665 }
666 }
667